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61.
Michel Michaelovitch de Mahiques Ilana Klein Coaracy Wainer Leticia Burone Renata Nagai Silvia Helena de Mello e Sousa Rubens Cesar Lopes Figueira Ilson Carlos Almeida da Silveira Mrcia Caruso Bícego Daniel Pavani Vicente Alves
yvind Hammer 《Quaternary International》2009,206(1-2):52
A high-resolution, multi-proxy record has been used to determine the environmental changes during the Holocene on the southern Brazilian shelf. Present oceanographic conditions reveal wind and freshwater input as the determinants of short-term productivity changes in the study area. Magnetic susceptibility and grain-size variations, together with proxies of productivity (organic carbon, carbon accumulation rate, Ba, Sr, and Ca content, Ba/Al, Ba/Ti, and Al/Ti ratios) were analyzed and compared with proxies of redox condition (V/Ti ratio), terrigenous input (Fe/Ca and Ti/Ca ratios), as well as other Element/Ti ratios, to evaluate the paleoceanographic and paleoclimatic changes over the period.The core covers a time interval of about 7650 years, with sedimentation rates varying from 0.025 to 0.250 cm a−1, which represent time intervals of between 8 and 80 a per sample. There is a clear change in the sedimentation rate at about 2800 B.P.All grain-size and elemental results indicate the occurrence of conspicuous changes between 5200 and 5000 cal. B.P., as well as between 3000 and 2800 cal. B.P. A comparison of the results with the palynological information available from the adjacent continental areas suggests that the sedimentary changes in this last interval may be correlated with the onset of modern climatic conditions in South America, and especially, with the onset of the Plata Plume Water, a water mass that carries cold, less saline waters towards the north. However, minor changes are observed at ca. 1500 B.P. and are correlated with an increase in the atmospheric humidity. Furthermore, a time-series analysis undertaken using several proxies indicated the occurrence of Sub-Milankovitch cycles, which may be compared with those reported worldwide. 相似文献
62.
Carlos J. Archanjo Patrick Launeau Maria Helena B. M. Hollanda José Wilson P. Macedo Dunyi Liu 《International Journal of Earth Sciences》2009,98(8):1793-1807
Anisotropy of magnetic susceptibility (AMS) applied to an alkaline granite from Meruoca (NE Brazil) recorded weak anisotropies,
typically below 4%, and a considerable dispersion of the AMS axes. Red-clouded feldspars and clots of metasomatic minerals
enclosed in magmatic crystals indicate that hydrothermal fluids altered the granite. U–Pb isotopic data show high-common Pb
on zircons but allowed the calculation of a mean SHRIMP age of 523 ± 9 Ma attributed to the magmatic crystallization. Growth
of fine oxides by late fluid–rock interactions was responsible for the scattering of AMS. Rock magnetic data indicate they
consist mainly of an oxidized magnetite and (titano)hematite. Shape preferred orientation of mafic aggregates measured in
granite quarries shows that the pluton preserves a gently dipping magmatic foliation. AMS in some quarries with a well-defined
magmatic fabric, however, remains highly dispersed. When AMS mimics the mafic shape fabric, only magnetic foliations share
a common orientation. Locally, AMS grounded in coarse Ti-poor magnetite associated with titanite develops a consistent subhorizontal
oblate fabric that agrees with tectonic models suggesting that the cupola of the pluton has been exposed by erosion. 相似文献
63.
Carlos Augusto Ramos e Silva Pablo Bezerra Dávalos Leonel da Silveira Lobo Sternberg Flavo Elano Soares de Souza Maria Helena Constantino Spyrides Paulo Sérgio Lucio 《Estuarine, Coastal and Shelf Science》2010
We evaluated the physical–chemical properties and nutrient concentrations in two shrimp ponds under conventional management, intensive (I-P) and semi-intensive (SI-P), and one under organic management (O-P), three days after loading and before stoking. The mean’s total alkalinity concentrations were 2.26 ± 0.04 mM, 2.28 ± 0.01 mM and 2.59 ± 0.01 mM, respectively for I-P, SI-P and O-P. Bicarbonate and carbonate accounted for 62% and 37% in I-P, 64% and 36% in SI-P and 83–17% in O-P, respectively. Aragonite and calcite were oversaturated around Ω = 5. Mean total phosphate (TP) concentrations were 441.37 ± 92.06 μg/L, 449 ± 48 μg/L and 473.64 ± 84.17 μg/L, under I-P SI-P and O-P management respectively. Following this sequence, NO3− concentration was 2.98 ± 0.7 μg/L, 1.16 ± 0.16 μg/L and 0.32 ± 0.12 μg/L, under I-P, SI-P and O-P management respectively. Thus, the data suggest that different management of farm organic waste leads to different chemical water quality. 相似文献
64.
Fernando O. Marques Fernando M. S. F. Marques António Mateus Jorge Figueiras Fernando M. Santos Rogério Mota Helena Amaral 《Environmental Geology》2005,47(8):1128-1137
A sodium bentonite is being used in the basal-isolation of landfills installed on strongly fractured granitic rocks of N Portugal (NW Iberia). To evaluate the performance of this clay as the ultimate impermeable basal barrier, a detailed study was carried out of the granite fracture network; the Na-bentonite was tested to characterize its mechanical and geotechnical behavior; geophysical surveys were made to search for landfill leakage and the waters found around the landfill were geochemically characterized. Fractures in the granite are usually open and devoid of mineral infillings or clay materials and, thus, permeability of the granite is very high. Dispersal of contaminants can be further enhanced by the position of the landfill on a high steep-sided ridge. Geophysical and geochemical data show evidence for seepage and dispersion of pollutants, which means that the clay failed as an ultimate isolation barrier against seepage. This investigation shows that this can be due to fracturing of the clay under load and/or to its non-homogeneous saturation and extreme shrinking character upon drying, which are accompanied by the formation of extensive cracking. Observations and experimental results suggest that the use of synthetic clays in the safe building of landfill bottom liners needs further research, and extreme care should be taken in preventing that clay water content suffers large variations after saturation, as this process considerably degrades the mechanical behavior and sealing properties of the studied Na-bentonite. 相似文献
65.
de Almeida EA Miyamoto S Bainy AC de Medeiros MH Di Mascio P 《Marine pollution bulletin》2004,49(5-6):386-392
Levels of antioxidant defenses and lipid peroxidation were evaluated in mussels exposed to lead (200 mg/l), iron (500 microg/l), cadmium (200 microg/l) and copper (40 microg/l), for 12, 24, 72 and 120 h. Glutathione S-transferase (GST) activity was unchanged with all treatments. Catalase (CAT) increased after 120 h of exposure to all metals. Mussels exposed to Cd for 12 h, and to Cu and Fe for 120 h had increased lipid peroxidation, which might be associated to decreased levels of reduced glutathione (GSH) and glutathione peroxidase (GPx) activity. Pb exposure caused GSH depletion after 12 h and increased GPx activity after 120 h. Negative correlations were observed between the enzyme phospholipid hydroperoxide glutathione peroxidase (PHGPx) and malonaldehyde (MDA) levels after Fe and Cu exposure, indicating a protective role of PHGPx against lipid peroxidation, and suggesting the use of this enzyme as a new potential biomarker of toxicity associated with contaminant exposure in mussels. 相似文献
66.
67.
The aim of the current work was to determine over 10 days the effects of water-borne exposure of the marine amphipod Gammarus locusta to 4 μgCu l−1 on the metallothionein (MT; measured by differential pulse polarography) protection system and lipid peroxidation (LP; thiobarbituric acid-reactive malondialdehyde equivalents) as a measure of oxidative damage. MT levels in exposed animals increased significantly at day 2 (36% > control; P<0.001) and remained high at days 6 and 10 (55 and 38%, respectively, P<0.001). The maximum level of MT at day 6 coincided with the highest Cu body-burden. LP increased within 1 day of exposure, indicative of Cu as an oxidative stressor. However, in contrast to MT, the highest LP level was seen at day 4 (68% > control, P<0.001) before returning to control values by day 6, indicating a protective role of MT against the pro-oxidant effects of Cu. 相似文献
68.
69.
Wesley R. Farnsworth Ólafur Ingólfsson Riko Noormets Lis Allaart Helena Alexanderson Mona Henriksen Anders Schomacker 《Boreas: An International Journal of Quaternary Research》2017,46(3):585-603
Evidence of a dynamic Holocene glacial history is preserved in the terrestrial and marine archives of St. Jonsfjorden, a small fjord‐system on the west coast of Spitsbergen, Svalbard. High‐resolution, remotely sensed imagery from marine and terrestrial environments was used to construct geomorphological maps that highlight an intricate glacial history of the entire fjord‐system. The geomorphology and stratigraphy indicate an early Holocene local glacier advance constrained to the Lateglacial–early Holocene transition. Identification and 14C dating of the thermophilous bivalve mollusc Modiolus modiolus to 10.0±0.12 cal. ka BP suggest a rapid northward migration of the species shortly after deglaciation. Further evidence enhances the understanding of the onset and subsequent climax of the Neoglacial‐Little Ice Age in inner St. Jonsfjorden. The present‐day terminus of Osbornebreen, the dominating glacier system in St. Jonsfjorden, is located over 8.5 km up‐fjord from its Neoglacial maximum extent. Cross‐cutting relationships suggest subsequent advances of all the smaller glaciers in the area following the break‐up of Osbornebreen. Glacial deposits, landforms and their cross‐cutting relationships observed in both terrestrial and marine settings imply a complex and highly dynamic environment through the later part of the Holocene. 相似文献
70.
Jakob Heyman Arjen P. Stroeven Marc W. Caffee Clas Hättestrand Jonathan M. Harbor Yingkui Li Helena Alexanderson Liping Zhou Alun Hubbard 《Quaternary Science Reviews》2011,30(15-16):1988-2001
The Bayan Har Shan, a prominent upland area in the northeastern sector of the Tibetan Plateau, hosts an extensive glacial geological record. To reconstruct its palaeoglaciology we have determined 10Be exposure ages based on 67 samples from boulders, surface pebbles, and sediment sections in conjunction with studies of the glacial geology (remote sensing and field studies) and numerical glacier modelling. Exposure ages from moraines and glacial sediments in Bayan Har Shan range from 3 ka to 129 ka, with a large disparity in exposure ages for individual sites and within the recognised four morphostratigraphical groups. The exposure age disparity cannot be explained by differences in inheritance without using unrealistic assumptions but it can be explained by differences in post-depositional shielding which produces exposure ages younger than the deglaciation age. We present a palaeoglaciological time-slice reconstruction in which the most restricted glaciation, with glaciers less than 10 km long, occurred before 40–65 ka. More extensive glaciations occurred before 60–100 ka and 95–165 ka. Maximum glaciation is poorly constrained but probably even older. The Bayan Har Shan exposure age dataset indicates that glaciers on the northeastern Tibetan Plateau have remained surprisingly restricted for at least 40 ka, including the global last glacial maximum (LGM). This case of a missing LGM is further supported by high-resolution glacier modelling experiments. 相似文献